Consequence Analysis for probable accidents of filter separators installed in Gas Pressure Reduction Stations

Mohammad Sadegh Yousefzadegan*, Amir Masoud Masoudi, Yaser Kazemi Ashtiani, Masoud

Kambarani Chemical Processes Design Group

ACECR (Tehran University branch) Tehran, Iran

yousefzadegan@jahat.ir

Seyed Iman Pishbin Research and Technology Department

Khorasan Razavi province Gas Company Mashhad, Iran

iman.pishbin@gmail.com

Abstract—Natural gas undergoes a number of processes as it is transported from wellhead to end-user, and many of these steps require filtration of the product. The filtration process removes debris and condensation from natural gas. Filter cartridges need to be periodically cleaned in order to perform efficiently and statistics show that the risk of fire in the cartridge cleaning process is high. As cartridge cleaning is performed during maintenance process, technicians and employees working near filter may encounter serious harms in case of fire together with environmental aspects. As a result, consequence assessment of the mentioned accident would be helpful in taking quicker actions and mitigating the harmful effects of fire for employees, equipments and environment. In this paper natural gas filter of Abbasabad gate station near Mashhad in NW of Iran is studied as a case. In order to predict the consequence of this accident, DNV Consequence Modeling software package is used for simulation. Necessary information for simulating the scenario is collected from reliable sources. Different weather categories comprised of cool, very cold, warm, very hot and windy weathers are defined to consider all meteorological conditions in which the accident might occur. Information on meteorological condition is gathered from the province meteorological organization. The hazardous accident is simulated by the software and the results of the simulation are analyzed. The severity of probable accidents is assessed consequently, and the safety distances around filter could be determined accordingly.

Keywords- Natural Gas; City Gate Station; Filter Separator; Consequence Modeling;

I. INTRODUCTION Natural gas produces less carbon dioxide when it is

burned than does either coal or petroleum. This has led governments into replacing other fossil fuels with natural gas. Statistics show that natural gas consumption increases by an average of 1.6 percent per year and that it will reach to 153 trillion cubic feet in 2030. In Iran, natural gas provides a great part of the nation's energy demand and its consumption increases by the rate of 12% per year which is much greater than the world's rate of increase. It is obvious that the increase in the mentioned energy supply consumptions is followed by the development of gas transmission and

distribution systems. Like any other process, environmental and safety issues are of great importance in these systems.

As natural gas consumption increases in the world, the importance of safety considerations in this field gets more highlighted. Management systems such as engineering codes, checklists and process safety management provide layers of protection against accidents. However, the potential for serious incidents cannot be totally eliminated. AIChE provides a quantitative method to evaluate risk and to identify areas for cost- effective risk reduction. Consequence analysis is an important stage in Chemical Process Quantitative Risk Analysis which evaluates the impacts of potential hazards [1].

Several valuable researches are performed on consequence and risk analysis of natural gas transmission and distribution systems. Spyros Sklavounos et al estimated the safe distance in the vicinity of fuel gas pipelines by using event tree analysis method and BREEZE software package [2]. Z.Y. Han and W.G. Weng worked on an integrated quantitative risk analysis method for natural gas pipeline network [3]. Their method is composed of the probability assessment of accidents, the analysis of consequences and the evaluation of risks. Many other works have been done on natural gas pipelines to improve safety (Krueger & Smith, 2003; Metropolo & Brown, 2004; Jo & Ahn, 2005; Jo & Crowl, 2008; Suardin, McPhate, & Sipkema 2009). Reviewing the papers in the field of safety in natural gas transmission and distribution systems show that all researches have only considered safety in gas pipelines while there are lots of other equipments that play important roles in the system and their failures could cause serious harms.

There was no previous work on consequence modeling of probable accidents in gas pressure reduction stations. As most of these stations are located near cities, it is very important for gas distribution companies to assess the impacts of their facilities on nearby inhabited areas, thought. To make it clear, distributers are generally willing to know the impact of probable incidents upon the nearby towns and environment with more precision in that inhabited areas are very close to city gate stations.

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2011 2nd International Conference on Environmental Science and Development IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore

Figure 1. Typical Filter separator used in city gate stations

TABLE I. BASIC INFORMATION OF ACCIDENT CONDITION

Parameters

Scenario

Type of scenario

Physical state of released material

Material released Pressure Tempe-

rature

- - (m3) (psig) (º C)

Filter Flash Fire / Filter Jet

Fire

Fixed time

release Gas 3333 3 20

Filter separators are inseparable parts of the natural gas processing system. Solids and/or fluids are removed from the gas in the filter/separator section. Investigations on past industrial accidents show that failure in filters could result in extreme financial losses and injuries. In 1996, failure in the closure of a 40-inch diameter filter separator resulted in an estimated $25 million in damages [4]. Similar accidents may occur in all filter separators. In this article Abbasabad gate station near Mashhad in North West of Iran is selected as a case and different steps of consequence modeling are performed on it in order to identify probable hazardous accidents of the filter and evaluate the undesirable consequences of these accidents.

II. GAS FILTRATION SYSTEM Natural gas filter units are installed at each station to

remove any entrained liquids and solids from the gas stream. The filters may comprise cyclonic elements to centrifuge particles and liquids to the sides of the enclosing pressure vessel. These particles and liquids will then drop down for collection in a sump, which can be drained periodically [5].

Depending on the gas quality and requested efficiency, a filter separator will be selected. A wide range of filter separators can be used, including a dust filter, a baffle plate / coalescer separator, cyclone separators, cyclone / coalescer separators and cyclone/cellulose separators. Each filter has a specific application area, separation efficiencies and operating range. The filter separators can be vertically positioned, or have horizontal configurations. Installed filter separators in Abbasabad gate station like most of filter separators used in Iranian Gas reduction stations are horizontal type (Fig. 1), so our calculation for probable accidents is based on this type of filters.

III. CONSEQUENCE MODELING Consequence analysis is an integral part of risk

assessment process which gives an estimation of the damages that a probable accident may bring to the properties and human beings. The consequence estimation scheme that is followed in this study involves three steps:

1. accident scenario selection 2. accident scenario modeling 3. accident impact assessment

The first step is selection of an accident scenario where possible accidents leading to hazardous consequences are

identified. The next step is accident scenario modeling. In the present study, DNV (Det Norske Veritas) consequence modeling software, PHAST 6.53.1 (Process Hazard Analysis Software Tool), is used to complete this step. After the second step is performed successfully, the results are analyzed in the third step and the production loss, human health, environmental aspects and safety loss together with assets loss will be estimated accordingly.

A. Accident Scenario Selection An accident scenario is a description of an expected

situation. It contains single events or combinations of them. The expectation of a scenario does not mean it will indeed occur, but that there is a reasonable probability that it would occur. A credible accident is defined as ‘the accident which is within the criteria of possibility and has a propensity to cause significant damage’. There may be a type of accident which can occur very frequently but would cause little damage. And there are other types of accident which may cause great damage but would have very low probability of occurrence in reverse. Both are not ‘credible’. But accidents which have appreciable probability of occurrence as well as significant damage potential (as quantified above) come under the category of ‘credible accidents’ [6].

As the primary step of consequence modeling, scenario selection plays an important role in the reliability of results. This paper investigates the consequences of the worst case scenario in a filter separator. This worst case scenario was selected in accordance with the identified hazards in the HAZOP study and considering the accident records of other Iranian cities gate stations. Accordingly, it is found out that flash fire and jet fire are likely incidents that are followed by the worst consequences. One of the most probable events that could lead to the fire is the back flow of natural gas in the rehabilitation time. Because the filter’s cap is opened during maintenance to allow the natural gas being released, oxygen in the air can enter the filter and react with Iron Sulfide accumulated on filter cartridge. As this reaction is exothermic, it can be a good start point for fire by providing the necessary initial heat.

B. Accident Scenario Modeling After the accident scenario is selected, it is modeled by

PHAST vr.6.53.1 to identify the possible consequences. In order to simulate the accident scenario and identify the consequences of the accident, PHAST requires some input data including process conditions at which the accident is

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TABLE IV. EFFECT ZONE DISTANCES OF FILTER FIX TIME RELEASE

Meteorological

condition

Jet fire affected distance (m)

Maximum distance affected by flash fire

Radiation Intensity LFL ½ LFL

kW/m2 4

kW/m2 12.5

kW/m2 37.5 (m)

Warm 42 32 25 28 52 Very hot 44 33.5 25.5 28 52

Cool 40 34 29 26 50 Windy 40 37 35 13 33

Figure 2. Radiation Intensity graph of Jet Fire in windy condition

TABLE II. FLAMMABILITY LIMITS OF NATURAL GAS Critical

limits

Material

Molecular Weight Mass %

Vapor Density (air=1)

LFL% UFL%

Methane 16.04 96.20 0.6 5 15 Ethane 30.1 0.98 1.05 3 12.5 Propane 44.1 0.15 1.52 2 9.5 i-Butane 58.12 0.05 2.06 1.8 8.4 n-Butane 58.12 0.10 2.05 1.5 9

n-Pentane 72.15 0.19 2.5 1.3 8

n-Hexane 86.18 0.32 3 1 7.7

CO2 44.01 1.10 1.53 - -

H2S 34.08 2.80E-06 1.19 4.3 45.5

N2 28.01 0.91 0.97 - -

occurred (pressure, temperature etc), the physical state of the released material (gas, liquid or both), physical, chemical and thermodynamic properties of the released material and topographical and meteorological conditions of the region. Although provision of all this information is a time consuming process, the more exact this information is the more realistic the results will be.

In this research all necessary information is collected from reliable sources. Table I shows the conditions at which the accident is occurred. This information is collected from the unit Process Flow Diagram.

In table II, physical and chemical property of Natural Gas which is required for modeling is presented.

As ground condition affects the dispersion process of the released material, it is required to gather information about the topographical conditions of the plant. This information is obtained from the satellite photos taken from the plant. Different topographical conditions are defined in PHAST. Having studied the satellite photos of the unit, it is found out that the most relevant definition in PHAST for Abbasabad gate station is regular large obstacle coverage areas (suburb- forest) for which the surface roughness is defined as 1m. Moreover, the official website of Khorasan Razavi meteorological organization is used for the relevant meteorological data. Five different conditions are identified for warm, very hot, cool, very cold and windy weather. This

information is summarized in table III. It should be noticed that due to harsh condition, maintenance operation would not be accomplished in very cold weather and this condition is omitted from the modeling.

Different mathematical models for three hazard categories including toxic gas dispersion, fires, and explosions are defined in PHAST. By entering all the necessary information, PHAST is able to run these models and to determine consequences of the chosen accidents. Analyzing the reports of the software, it is concluded that the most destructive consequences of the filter fixed release

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TABLE III. METEOROLOGICAL CONDITION OF THE PLANT

Para-

meters

Weather

Ambient

tempe-

rature

Relative

humidity

Maximum

wind

speed

Wind

direc-

tion

Atmos-

pheric

stability

(ºC) % (m/s) (º) -

Warm 23 38.42 2.94 13 C

Very hot 40.6 0 2.94 13 B

Cool 8 59.17 4.97 13 E

Very

cold-21 99 4.97 13 D

Windy 23 38.42 23.9 13 D

Figure 4. Side view for flash Fire area in hot weather

Figure 3. Effect zone of Jet Fire in windy condition Figure 5. Effect zone of Flash Fire in hot weather

scenario are related to jet fire and flash fire. Effect zone distances from the center of the accident are presented in table IV.

Fig. 2 shows radiation intensity of jet fire in the wind direction at windy condition. Effect zones diagram for jet fire which is fitted with the region map is illustrated in fig. 3.

Fig. 4 shows a side view for probable flash fire in hot weather. Effect zones diagram for the flash fire which is fitted with the region map is illustrated in fig. 5.

C. Accident Impact Assessment Jet fire in the filters does not exceed the plant boundary,

but can harm people in the plant. It should be mentioned that jet fire is extended at the wind direction. Although regular wind direction has been shown in the map, we should consider the affected area in all directions.

Radiation intensity of jet fire can be more understandable by the following examples:

• 4 kW/m2 : Staffs feel pain after 20 second. It can cause blister at the skin.

• 12.5 kW/m2 : it is the minimum radiation required for firing wood and melting the plastic materials.

• 37.5 kW/m2 : this is adequate radiation to damage equipments

Flash fire effect zone diagrams show that there will be flammable concentrations of natural gas in the plant area. When this flammable bulk of gas reaches to a source of ignition, there will be flash fire. Flash fire flames will cause extreme damages to the equipments besides serious injuries to the employees and in the worst case, especially at the maintenance time, can claim lives.

It should be also noticed that if fire occurs in the filters of a gate station, there is possibility of explosion or other kinds of accidents in nearby equipments which could create a catastrophic condition by domino effect.

IV. CONCLUSION This paper discusses the procedure for consequence

evaluation of the possible accidents in natural gas filtering

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systems. Natural gas may pose major risks to the surroundings because of its flammability and toxicity. Making confident predictions about the consequences of the probable accidents in case of natural gas release could help us define safety distances around filter separators. In this paper Abbasabad gate station filter separator is studied as the case. The results of this study could be summarized as follows:

• Fixed time release of natural gas from filter is selected as the worst case scenario both because it will have the worst consequences and that it is probable enough to be identified as a credible scenario.

• Results of the simulation with PHAST vr.6.53.1 indicate that the most significant consequences for the natural gas release are flash fire and jet fire.

• Comparing the results of the simulation for jet fire in different meteorological conditions, we realized that jet fire will travel the furthest distance in windy condition. This distance is not exceed plant’s boundary but is considerable as it can harm personnel and/or affect on other equipments in the station.

• A valid comparison between the results of simulation for flash fire in different meteorological conditions shows that flammable concentrations of natural gas will encompass the largest area in hot weather. Moreover, it is concluded that if the flammable concentrations of natural gas are ignited,

main equipments will be seriously damaged and employees in the unit could suffer considerable harms; although nearby farmlands won’t be extremely destructed.

ACKNOWLEDGMENT We highly appreciate Khorasan Razavi Gas Co. for their

technical and financial support of this research.

REFERENCES [1] Center For Chemical Process Safety of the American Institute of

Chemical Engineers, Guidelines For Chemical Process Quantitative Risk Analysis, 2nd edition, 2000

[2] Sklavounos, S., Rigas, F., 2006, Estimation of safety distances in the vicinity of fuel gas pipelines, Journal of Loss Prevention in the Process Industries, Volume 19, pp. 24–31

[3] Han, Z.Y. and Weng, W.G., 2010, An integrated quantitative risk analysis method for natural gas pipeline network, Journal of Loss Prevention in the Process Industries, Volume 23, pp. 428-436.

[4] Project Findings for the Investigation of the Explosion of High Pressure Natural Gas Filter Separator at McDonald Island, California, APTECH Report 93102025-4-5 (October 1996).

[5] Mokhatab, S., Poe, W., and Speight, J., Handbook of natural gas transmission and processing, Gulf Professional Publishing, 2006, pp.417

[6] Khan, F. and Abbasi, S.A., 2002, A criterion for developing credible accident scenarios for risk assessment , Journal of Loss Prevention in the Process Industries., Volume 15, pp. 467-475

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